1. Field of Invention
This invention pertains to a combustible aerogel or other nanocellular substrate having an impregnant on its internal and external surfaces that reacts spontaneously upon exposure to air to produce infrared and/or visible radiation and a process for its preparation.
2. Description of Related Art
Solid materials that spontaneously react exothermically with air, thereby emitting significant infrared or visible radiation fall primarily into two categories The first is fine powders of various elements, mostly metals, including iron, nickel, aluminum, magnesium, calcium, lithium, sodium, potassium, phosphorous, zirconium and titanium, and their alloys. The second category is activated metal foils, including iron, nickel and cobalt.
A number of techniques exist to produce metal powders including milling, atomization, reduction of metal oxides or solutions of metal salts, and decomposition of metal carbonyls. Some metals, such as iron, can be prepared as extremely fine powders by segregating Fe(II) in reverse micelle solutions and reducing to metal by exposure to hydrazine. Raney metal powders, such as iron or nickel, are synthesized by leaching Al from an alloy of Ni or Fe and Al.
For some applications, it is desirable to have a bulk material rather than powders. While powders can be mixed with binders or imbedded in material such as carbon cloth to form a bulk, an alternative is to activate the surface of bulk metals. Metals, such as iron, nickel or cobalt, can be activated by a diffusion and leaching process yielding a highly porous surface layer, thereby increasing the area of metal surface available for oxidation and hence the amount of heat generated. Typically, activated metals are prepared in a two-dimensional foil to minimize the volume of non-reactive material. The duration of IR emission can be tailored from about 3 seconds to about 30 seconds by adjusting the size and form of the activated metal foil.
This patent relates to a new type of solid material that spontaneously emits infrared and/or visible radiation upon exposure to air. These materials comprise a highly porous, combustible substrate combined with an impregnant on the internal and external surfaces that generates sufficient heat upon exposure to air to initiate the combustion of the substrate. These materials have several unique properties. They are extremely lightweight and are distinctly superior to existing materials in terms of the duration of emission, which can be very long (about 30 minutes) or very short (about 30 seconds), depending on the size of the substrate. They are frangible, allowing them to be stored as large monoliths, yet readily broken into smaller pieces during deployment. In a preferred embodiment, the substrate is a combustible aerogel or other porous nanocellular material.
Aerogels are a class of materials with extremely low density, high porosity and high surface area. Their physical properties result from their structure, which consists of nanometer-scale solid particles that are connected to form a three-dimensional, mesoporous network Aerogels are generally prepared by synthesizing a sol-gel with a large volume fraction of liquid and then removing the solvent from the pores supercritically in order to avoid large capillary forces and shrinkage during drying. A range of compositions can be prepared as aerogels, including silica, alumina, zirconia, titania and several organic compounds, including carbon, resorcinol/formaldehyde and melamine/formaldehyde.
A new proprietary process has recently been developed for preparing organic nanocellular materials that have physical properties similar to aerogels. This proprietary process uses modified parameters during the sol-gel reaction, which strengthens the gel structure such that it can be dried under ambient conditions without significant collapse or shrinkage of the pores. The resulting material has physical properties that are similar to the supercritically dried aerogel form.
Combustible porous, nanoscale material can also be prepared by adding a combustible powder during the synthesis of a non-combustible aerogel. In order to avoid coating the powder with the non-combustible phase and thereby inhibiting the burning of the combustible component, the powder is commingled with the sol shortly before gelation occurs either by briefly mixing or pouring the sol over the dry powder. After supercritical drying, the composite has the physical properties of the single phase, noncombustible aerogel and the combustible phase is accessible via the interconnected porosity.